Did they run a full thyroid panel (not just TSH)? Do you know what your actual #s are?
Some info to talk with your doctor about: Does Your Doctor Know About the New TSH Lab Standards?
That site actually has a lot of really good info.
Do you have a lot of the traditional hypothyroid symptoms?
More info here: My TSH Test Results Are Normal, But I Still Have Symptoms

I don't know how old you are, but perimenopause can also be a factor. That can start in your 30's.

I assume the doctor checked your iron and the levels of other vitimains and minerals?

For me, I was experiencing a lot of hair loss and my TSH was just under 3. Some endocrinologists say under 2 is better for women. I asked and got my doc to prescribe a low dose of thyroid hormone, I'm also 45 and definitely perimenopausal (oh the joys). I also use an OTC progesterone cream from Amazon and both have helped a lot.

Good luck!
CG since spring '11 and still learning
2c/3a (I think), high porosity, drugstore dyed (hate my gray!)